A fuel cell is an electrochemical device to convert chemical energy of fuel gas such as hydrogen gas and oxidant gas containing oxygen supplied thereto, directly to electric energy which is extracted from electrodes provided on both sides of an electrolyte thereof. A fuel cell using a solid polymer electrolyte membrane has low operation temperature and can be easily handled, and therefore, it has been a focus of attention as a power source for an electric vehicle.
The solid polymer electrolyte membrane is required to be retained in a moderately humidified state in order to exert oxidant gas or the both gases are humidified to be supplied to the fuel cell. Thus, water added for the humidification and water produced by a power generation reaction in the fuel cell are condensed and may cause clogging or blocking of a gas passage in the fuel cell, depending on operating conditions of the fuel cell.
Moreover, in a fuel cell using air as the oxidant gas, nitrogen contained in the air passes through a solid polymer membrane thereof, and accumulates in a fuel gas circulation system. Consequently, a fuel gas partial pressure decreases at a fuel electrode of the fuel cell, lowering operation efficiency thereof. In order to resolve the clogging and the accumulation of nitrogen, purging is performed for the fuel gas circulation system.
Japanese Patent Laid-Open Publication No. 2002-243417 discloses a fuel cell system which removes impurities accumulated in a hydrogen gas circulation system by opening a purge valve provided in the system and releasing anode off-gas to outside of the system.